Deacy-Style Amps

Ever since reading the story about the famous Brian May’s “Deacy” amp many years ago, I’ve wanted to try doing something similar. For those of you who are not familiar, the legend has it that John Deacon (bass player of Queen) found a mysterious circuit board in a dumpster and converted it to a small combo guitar amp for May that was used to record many of the greatest Queen songs. Some fellow nerds analyzed the circuit and discovered that it came from Supersonic Monarch PRG80 turntable radio. Supposedly, amplifier design from the PRG80 is very similar to the standard Mullard reference design for low power Germanium amplifier from the era that features one preamp transistor, one transistor that drives the phase splitter transformer, two output transistors in push/pull configuration and an output transformer. Speaking of which, I suggest checking out the Mullard – Reference Manual of Transistor Circuits from the 1960, the circuit in question is on page 171, although there’s more interesting information to read there.

mullard_amp

There are plenty of vintage transistor radios, phonographs and record/cassette players from 60s and 70s that follow the same topology and if you don’t care that they are in working condition, they can be bought for close to nothing. Back in the days everybody made transistor audio devices and most of them are pretty similar. Japan made a bunch (Toshiba, Panasonic, Aiwa…), usually with Japanese 2SBxx transistors, Russians built them using their own transistors, Germans and Dutch had a few brands and pretty much every east European country made similar radio circuits (some are even made in my town), some with Russian, others with western world transistors.

I browsed local classifieds and narrowed down my selection to a handful of cheap old devices, all less than 10 bucks each. To determine whether or not a deviceis good candidate for salvaging, I would try to find the schematics or gut shots (the awesome RadioMuseum.org helped tremendously) and confirm that its amplifier topology is correct. I decided to try several different variations and find the one I like the most – Russian, Japanese or Western transistors, push pull or single-ended, with or without the transformers, etc.

To power my radio amplifiers, I built a simple, yet flexible regulated variable voltage supply and Dallas Rangemaster booster that would allow me to shape the sound of guitar and hit the amp a bit harder if I want to. What’s common for pretty much all of these is that they are not really tailored as guitar amps. Input impedance of audio amplifiers is usually lower than typical guitar impedance and they do not cut bass between the gain stages like guitar amplifiers do, so they can sound “farty” and “boomy” when overdriven. Rangemaster at the front helps shape the input signal, cut the excess bass and hit the amplifier harder if needed.

Below is the list of radios/magnetophones I played with:

Tech Tip: a quick test to verify if old capacitors still work (more or less) is to measure resistance across them with a DMM set to 2M or similar large value. If the capacitor gets charged by the voltage of the DMM, the resistance should slowly raise. If it doesn’t change, it’s probably dead. Alternatively, connecting the capacitor to a battery for a few seconds and measuring voltage across the leads immediately after disconnecting it should show if it is charged. DMM should read *some* voltage and it will drop to 0 after some time. But it all 50+ years old devices, it’s best to just replace all electrolytic capacitors with good quality new ones and not worry about them for decades to come.

Comments
5 Responses to “Deacy-Style Amps”
  1. B. Guitar says:

    Does anyone know of a suitable transformer pair to make this circuit? I’m looking for impedance numbers, ratios, etc.

    Thanks!

    -Brian

    • bancika says:

      I’ve heard that a pair of Xicon 42TM024-RC (driver) and 42TU048-RC (output) transformers should do the job. Another option are Eagle LT44 (driver) and LT700 (output).
      One of those two options should be available through major suppliers, like Mouser.

  2. L. Kali says:

    Little extra note:
    The electrolytic Max rate is 25V,so I guess the 22.5V supply is adequate.
    Both channel sound level are the same. Output transistors using 4 pieces of NEC 2SD150.Thanks!

  3. L. Kali says:

    Hi,
    I have a similar style of amplifier.
    Denon CMS-400.I am facing with sound output issues. I need to crank up the volume all the way to the max. In order to get some sound. The sound is too sharp, slight bass. Checked the power supply to the output transistor section is there around 22.5V.
    Any idea what could be issue.
    Pls advice. Thanks

    • bancika says:

      Could be a dry electrolytic if it’s an old device. I’m not familiar with that amplifier, suggest posting on some forum where people have more experience fixing broken amps. http://www.diyaudio.com forum is a good place to start.

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    The idea behind this site is to share my experience with Do It Yourself approach to guitars, amplifiers and pedals. Whether you want to save a couple of bucks by performing a mod or upgrade yourself instead of paying a tech, or want to build your own piece of gear from scratch, I'm sure you will find something interesting here. Also, this is the home of DIY Layout Creator, a free piece of software for drawing circuit layouts and schematics, written with DIY enthusiasts in mind.